1. Field of the Invention
The present invention relates to the electrodeposition of metal, and more particularly to the metallization of flexible polymer sheets. The invention is particularly applicable to a process and apparatus for electroplating a metal layer onto a non-metallic electrically insulating substrate with a flash of metal adhered thereon.
2. State of the Art
The electrodeposition of metals from an aqueous solution is well known in the art. Simply stated, the process involves the use of a cathode, an anode (collectively called "electrodes"), an aqueous solution containing ions of the metal to be electrodeposited and an external current source. As electrical current is furnished to the anode, the metal ions are reduced and electrodeposited from the aqueous solution. Practically any metal which can be solvated by water (typically metallic salts) can be electrodeposited by the above-defined apparatus.
Electrodeposited copper is used widely in the electronics industry. Traditionally, copper is electrodeposited in rolls, cut to sheets and bonded to polymeric boards and etched. Then, discrete electronic parts are attached to the circuit board and the circuit board is inserted into an apparatus or device.
When the non-metallic, electrically insulating substrate is a flexible polymeric sheet, the metal, such as copper, may be electrodeposited directly on a flash of metal which has been sputtered, vapor deposited, electrolessly deposited, or adhered by similar techniques on the flexible polymeric substrate. Such an approach obviates the need for the intermediate step of bonding a metal foil to the substrate. The flexible polymeric sheet may be pretreated prior to depositing the flash of metal thereon. Once the polymer is initially metallized, metal may be electrodeposited on the metal flash yielding thicknesses of electrodeposited metal up to conventional thicknesses, i.e. of from about 0.25 oz. to about 2 oz. (corresponding to thicknesses of about 0.3 mils to about 2.8 mils of electrodeposited metal).
The resulting flexible, metal coated polymeric films find utility in flex circuits, tape automated bonding, electromagnetic interference shielding and other fields where metalized substrates are useful.
The following U.S. patents describe inventions relating to the metallization of polymers and other such non-metals.
Morrissey et al., U.S. Pat. No. 4,683,036, describe a method of electroplating a nonconductive substrate utilizing a photoresist and the reductive capacity of hydrogen in the presence of a metallic catalyst, the catalyst located on the substrate to be coated with metal.
Pian et al., U.S. Pat. No. 4,897,164, describe a method of electroplating the walls of through holes in laminated printing boards.
Bladon, U.S. Pat. No. 4,919,768, describes a method of electroplating an article of manufacture.
Pendleton, U.S. Pat No. 5,015,339, describes a method of electroplating a metal layer to the surface of a nonconductive material.
Bladon et al., U.S. Pat. No. 4,952,286, describe a method for plating the surface of a nonconducting article.
Beach et al., U.S. Pat. No. 4,673,469, describe a method and an apparatus for depositing metal on articles involving initially an autocatalytic process followed by an electroplating step.
Houska et al., U.S. Pat. No. 4,322,280, describe an electrolytic device for the electrodeposition of a metal on at least one surface of a tape which has been previously coated with a metal on that surface.
Goffredo et al., U.S. Pat. No. 4,576,685, describe a process and apparatus for the deposition of metal on generally flat surfaces through an electroless deposition process followed by an electrodeposition process.
Deyrup, U.S. Pat. No. 3,963,590, describes a process of pre-etching, etching, neutralizing and treating the surface of polyoxymethylene for electroless deposition of a metal followed by an electroplating step.
Conventional electrodeposition methods for copper on flexible polymeric sheets use current densities of from about twenty-five to about fifty amps per square foot. These current densities result in lengthy deposition times especially when thicknesses of greater than one mil of copper are desired. In this respect, the typical amount of electrodeposited copper on flexible polymeric sheets is typically referred to in "ounces." One ounce is the weight of copper for a one square foot of copper sheet (this represents a thickness of, on the average, 1.35 mils of copper). With conventional electrodeposition methods known heretofore, about forty to sixty minutes is required to electrodeposit one ounce of copper onto one square foot of a flexible polymeric sheet.
The rate of metal deposition in such an electrodeposition process is basically dependant upon the current which can be applied to the metal on the polymer substrate, which metal in effect becomes a conductor for the current. In one respect, the current to the web is limited by the thickness of the metal on the substrate, as well as by the current-carrying characteristics of the metal on the substrate. In another respect, the current applied to the metal substrate is determined by the anode design and arrangement, particularly the current density which can be generated at the anode surface(s) and the power loss in heat generated during the electrodeposition process.
Methods and apparatuses known heretofore are generally limited because of their designs in the amount of current which can be applied to the polymeric substrate and are limited in another respect in that the current applied to the substrate is based upon the thickness of initial metal flash on the substrate.
The present invention overcomes the limitations of apparatus known heretofore, and provides a method and apparatus for electrodepositing a metal onto a non-metallic, electrically insulating substrate, which apparatus and method dramatically reduce the electrodeposition time by reducing the gap between the active anode surfaces and the moving substrate thereby reducing the thermal power loss through a reduction of voltage, by increasing the current density which can be applied to the active anode surfaces, and by utilizing the current carrying capacity of the deposited metal to facilitate application of higher current to the substrate.